by Arun Vemuri, General Manager, Automotive Body Electronics & Lighting at Texas Instruments
When it comes to comfort and convenience in automotive, body motors are the key, whether it’s a perfectly positioned driver’s seat or the smooth, simple operation of doors, windows, and other vehicle parts.
The purpose of these applications varies greatly, but one commonality is movement. Every system designer knows that the motion must be smooth, precise, and easy for drivers and passengers to engage. To that end, electric motors make moving these mechanical components effortless.
The electric motors used in body motor systems are controlled using switches arranged in the letter “H.” This circuit configuration, shown in Figure 1, is called an H-bridge. Relays are one way to implement the switches in an H-bridge; however, relays tend to make a clicking noise when the contacts are engaged or disengaged and cannot achieve precise motor position or speed control. That’s why designers are increasingly turning to metal-oxide-semiconductor field-effect transistor (MOSFET) switches in H-bridges.
Figure 1: An H-bridge circuit configuration
The switch to MOSFETs
Using MOSFETs as switches enhances the control of electric motors but presents new technical challenges in electronic control module design, including:
- How to suppress electromagnetic interference (EMI) from pulse-width-modulated (PWM) signals.
- Thermal management to address heat generated from the current.
- Current sensing to detect malfunctions and determine position and commutation.
- Power-off braking to prevent damage to electronic components.
- Diagnostics and protection to detect circuit continuity faults such as open and short circuits.
Because implementing circuits to address these challenges could increase solution size. This article will describe new integrated circuit (IC) motor drivers that reduce body motor design complexity and development times. These motor drivers integrate analog features such as PWM edge slewing, current sensing, and power off braking and are developed with a state-of-the-art analog IC fabrication process. These analog features are integrated into motor drivers used to drive:
- Brushed direct current (BDC) motors, which commutate using mechanical systems. You’ll find BDC motors in seats, roofs, doors, and windows.
- Brushless direct current (BLDC) motors, which use electronics to achieve commutation. BLDCs motors can be found in power trunks.
Using an H-bridge to mitigate EMI
Driving MOSFETs in an H-bridge using PWM signals results in EMI. Control module designers use filters, optimal layouts and PWM edge shaping to mitigate EMI.
H-bridge gate-driver products for BDC motors and three-phase gate drivers for BLDC motors all integrate smart gate-drive technology. This technology is specifically used to control the shape of the PWM rising and falling edges and offers the flexibility to choose a shape that reduces EMI while not degrading the MOSFETs’ thermal performance.
Dual half H-bridge BDC motor gate-driver products can control loads such as window and roof motors, while a multi-half H-bridge is a good fit for seat applications that typically have multiple motors.
Choosing motor drivers to cool thermals
An electric motor’s current flows through the MOSFETs in an H-bridge. Since the MOSFET switches have resistance, the current flowing through these switches generates heat. If you don’t remove this generated heat from the vicinity of the MOSFET, its temperature could rise such that it begins operating outside of its safe operating area.
For high current loads, gate-driver products give you the choice of implementing your designs using discrete MOSFETs. Electronic control module designers can optimize the gate driver’s placement and layout and the discrete MOSFETs and achieve optimal thermal management. For low load current loads, devices with integrated H-bridge MOSFETs can be used to drive the loads while achieving optimal thermal performance. Integration of MOSFETs decreases the overall solution size. Note that integrated devices with H-bridge MOSFETs have different maximum current ratings and can drive low-current loads such as mirror X/Y motors.
In applications such as seat cooling fans, low-current, three-phase BLDC motors are used. In such cases, a three-phase BLDC motor driver with integrated MOSFETs can be used. The three-phase integrated motor drivers also integrates the commutation algorithm. These integrated BLDC drivers can drive ventilation fans in seats.
Protecting the system with current sensing
Measuring the current flowing in electric motors is used to detect motor malfunctions, determine motor position, and perform motor commutation. One common method to measure current in an electric motor is to use a resistor in the current flow path and then amplify the voltage across the resistor using an amplifier.
BDC and BLDC motor gate-driver products should integrate a current-sense amplifier to amplify the voltage across the resistor. Some devices even offer an in-line current-sense amplifier; that is, a single current-sense resistor can measure the current flowing in the motor regardless of motor rotation direction. This contrasts to a current-sense amplifier, which is used to measure the low side of an H-bridge and cannot infer the direction of motor rotation.
One common use of in-line current sensing is to measure BDC motor ripple. The measured in-line current is post-processed using either an analog circuit or software to infer the motor position, which is in turn used to control and regulate window, roof, trunk, or seat positions.
System designers are considering the use of BLDC motors for applications such as windows in electric vehicles because of the low acoustic noise of BLDC motors and seat base rotation in autonomous vehicles because of the ability of BLDC motors to drive high power loads. In such applications, the motor is also measured to monitor the motor for faults. BLDC motor drivers also integrate current-sense amplifiers, allow designers to optimize their control module design.
Preventing damage with power-off braking
When a car is turned off, and a mechanical system that has a motor coupled is moved, a BDC electric motor becomes an electric generator. For example, consider a driver that closes an electric motor-driven trunk manually rather than pressing the button.
When the driver closes the trunk manually, the electric motor used to move the trunk starts rotating. This rotation causes the electric motor to function as a generator, thus generating a current that flows into the control module. This generated current could cause decoupling capacitors connected to the H-bridge supply in the control module to charge to high voltages, which in turn could lead to damage to not only the decoupling capacitor but also other electronic components connected to the H-bridge supply trace in the control module
Gate drivers targeted for trunk control module applications integrate a power-off braking feature, which measures the voltage being generated and applies electronic brakes to the motor. This feature stops the motor from rotating, which in turn stops the generation of current.
Integrating diagnostics and protection
BDC and BLDC gate drivers should integrate diagnostic circuits to detect circuit continuity faults such as open and short circuits. These circuits will prevent the MOSFETs from turning on in the presence of faults, thus protecting the control module from damage.
In addition to integration, products that offer failure-mode distribution and pin failure-mode analysis information can help you design a control module in compliance with the International Organization for Standardization 26262 automotive functional safety standard.